Semiconductor materials that include compounds of copper indium diselenide (CIS) with gallium substituted for all or part of the indium, commonly referred to as copper indium gallium diselenide (CIGS), are used in many photovoltaic devices. Importantly, CIGS semiconductor materials have a direct band gap that permits strong absorption of solar radiation in the visible range. CIGS semiconductor materials are therefore often used as absorber layers in thin-film solar cells. As a result, CIGS solar cells have demonstrated high efficiencies and good stability as compared to other common absorber layer compounds such as cadmium telluride (CdTe) and amorphous silicon (a-Si).
Solar cell devices typically include one or more of a substrate, barrier layer, back contact layer, semiconductor layer, buffer layer, intrinsic transparent oxide layer, and conducting transparent oxide layer. In a solar cell device the CIGS materials used for photovoltaic conversion need to have a p-type semiconductor character and appropriate charge transport properties. The charge transport properties of the CIGS materials are related to the crystallinity of the material. It is therefore important that the CIGS material is at least partially crystallized in order to have sufficient charge transport properties for use in solar cells.
CIGS thin-films can be deposited by various techniques, which are typically vacuum based. One technique involves the use of precursors. In this technique, intermediate compounds are used and have physicochemical properties that are distinct from those of CIGS and make them incapable of photovoltaic conversion. The precursors are initially deposited in a thin-film form, and this thin-film is subsequently processed to form the intended CIGS layer. When precursor materials are deposited at a low temperature, the resulting CIGS thin-films are weakly crystallized or amorphous. These thin-films need to be annealed by supplying heat to improve the crystallization of the CIGS and provide satisfactory charge transport properties. At the temperatures that allow at least partial crystallization of the CIGS, however, one of the constituent elements of the CIGS, namely the selenium, is more volatile than the other elements. It is therefore difficult to obtain crystallized CIGS with the intended composition and stoichiometry without adding selenium during annealing of the precursor layer. In the fabrication of CIGS thin-films for photovoltaic applications, therefore, time consuming annealing of the precursor deposits in the presence of a selenium excess in the vapor phase is needed.
Another technique for depositing CIGS thin-films involves vacuum evaporation. Devices formed by this technique often have high photovoltaic conversion efficiencies compared to techniques that use precursor materials. Typically, co-evaporation of the copper, indium, gallium, and selenium is performed in the presence of a substrate. This co-evaporation technique has an advantage in that the content of gallium in the thin-film light-absorbing layer can be regulated to optimize the bandgap. Evaporation is a technique that can be difficult to use on the industrial scale. In particular, it is challenging to provide uniform thin-films over large surface areas, such as those used for fabrication of solar cells. Efficient use of the primary materials is also challenging. Selenium is particularly difficult to use efficiently because of its high vapor pressure.